LHCb

Abstract:

A study on the Bose-Einstein correlations for triplets of same-sign pions is presented. The analysis is performed using proton-proton collisions at a centre-of-mass energy of s = 7 TeV, recorded by the LHCb experiment, corresponding to an integrated luminosity of 1.0 fb−1. For the first time, the results are interpreted in the core-halo model. The parameters of the model are determined in regions of charged-particle multiplicity. This measurement provides insight into the nature of hadronisation in terms of coherence, being consistent with the presence of coherent emission of pions.

Abstract:

Using $e^{+}{e}^{-}$ collision data collected with the BESIII detector operating at the Beijing electron positron collider, the cross section of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ is measured at 59 points with center-of-mass energy $\sqrt{s}$ ranging from 4.009 to 4.950 GeV with a total integrated luminosity of $22.2{\mathrm{fb}}^{-1}$ . The cross section between 4.3 and 4.45 GeV exhibits a plateaulike shape and drops sharply around 4.5 GeV, which cannot be described by two resonances only. Three coherent Breit-Wigner functions are used to parametrize the $\sqrt{s}$ -dependent cross section line shape. The masses and widths are determined to be $M_1=({4223.6}_{-3.7-2.9}^{+3.6+2.6})\mathrm{MeV}/c^2$ , ${\mathrm{\Gamma }}_1=({58.5}_{-11.4-6.5}^{+10.8+6.7})\mathrm{MeV}$ , $M_2=({4327.4}_{-18.8-9.3}^{+20.1+10.7})\mathrm{MeV}/c^2$ , ${\mathrm{\Gamma }}_2=({244.1}_{-27.1-18.3}^{+34.0+24.2})\mathrm{MeV}$ , and $M_3=({4467.4}_{-5.4-2.7}^{+7.2+3.2})\mathrm{MeV}/c^2$ , and ${\mathrm{\Gamma }}_3=({62.8}_{-14.4-7.0}^{+19.2+9.9})\mathrm{MeV}$ . The first uncertainties are statistical and the second are systematic. The inclusion of the relatively narrower third component proves crucial for reproducing the drop at around 4.5 GeV. The statistical significance of the three-resonance assumption over the two-resonance assumption is greater than $5\sigma$ .

Abstract:

By analysing e+e− annihilation data corresponding to an integrated luminosity of 368.48 pb−1 collected at the centre-of-mass energies of s = 4.918 and 4.951 GeV with the BESIII detector, we report the first observation of Λc(2595)+ and Λc(2625)+ → Λc+π0π0 with statistical significances of 7.9σ and 11.8σ, respectively. The branching fractions of Λc(2595)+ and Λc(2625)+ → Λc+π0π0 are measured to be (59.5 ± 11.1stat. ± 7.9syst.)% and (41.0 ± 5.2stat. ± 3.3syst.)%, respectively. The absolute branching fraction of Λc(2595)+ is consistent with the expectation of the mechanism referred to as the threshold effect, proposed for the strong decays of Λc(2595)+ within uncertainty.

Abstract:

Using $20.3{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at a center-of-mass energy of $E_{\mathrm{c}\mathrm{.}\mathrm{m}\mathrm{.}}=3.773\mathrm{GeV}$ with the BESIII detector operating at the BEPCII collider, we determine the branching fraction of the leptonic decay $D^{+}\to {\mu }^{+}{\nu }_{\mu }$ to be $(4.034\pm 0.080_{\mathrm{stat}}\pm 0.040_{\mathrm{syst}})\times {10}^{-4}$ . Interpreting our measurement with knowledge of the Fermi coupling constant $G_F$ , the masses of the $D^{+}$ and ${\mu }^{+}$ as well as the lifetime of the $D^{+}$ , we determine $f_{D^{+}}|V_{cd}|=(48.02\pm 0.48_{\mathrm{stat}}\pm 0.24_{\mathrm{syst}}\pm 0.12_{\text{input}}\pm 0.15_{\mathrm{EM}})\mathrm{MeV}$ after taking into account necessary radiative corrections. This result is a factor of 2.3 more precise than the previous best measurement. Using the value of the magnitude of the $c\to d$ Cabibbo-Kobayashi-Maskawa matrix element $|V_{cd}|$ given by the global standard model fit, we obtain the $D^{+}$ decay constant $f_{D^{+}}=(213.5\pm {2.1}_{\mathrm{stat}}\pm {1.1}_{\mathrm{syst}}\pm {0.8}_{\text{input}}\pm {0.7}_{\mathrm{EM}})\mathrm{MeV}$ . Alternatively, using the value of $f_{D^{+}}$ from a precise lattice quantum chromodynamics calculation, we extract $|V_{cd}|=0.2265\pm 0.0023_{\mathrm{stat}}\pm 0.0011_{\mathrm{syst}}\pm 0.0009_{\text{input}}\pm 0.0007_{\mathrm{EM}}$ .